1. Field of the Invention
The present invention relates to a heating apparatus which is preferably used as a fixing apparatus of an image forming apparatus such as a copying machine or a printer. The fixing apparatus heats a recording material, on which a toner image made of a heat-softening resin is formed and carried, to form a permanently-fixed image on the recording material.
More particularly, the present invention relates to a heating apparatus of a type for heating a surface of a rotary member for heating the heating material (the material to be heated), the apparatus including: the rotary member for heating the heating material; an opposing member that forms a nip with respect to the rotary member; a heating member for heating a portion of a rotary member surface other than the nip; and temperature control means for controlling temperature used for heating the rotary member by the heating member, in which the heating material inserted in the nip to be nipped and conveyed therein using heat of the rotary member. The present invention also relates to an image forming apparatus including the heating apparatus as a fixing apparatus.
2. Related Background Art
For convenience, a description will be made taking as an example an image forming apparatus such as an electrophotographic copying machine, a printer, or a fax machine.
A fixing apparatus of an image forming apparatus is used for a process of heat-fixing an unfixed toner image to a surface of a recording material as a permanently-fixed image. The unfixed toner image is obtained in an image forming part of the image forming apparatus appropriately by means of an image forming process such as electrophotography, electrostatic recording, or magnetic recording. The unfixed toner image is formed on the surface of the recording material by using toner (a visualizing agent) composed of a heat-fusing resin or the like in a direct method or an indirect (transferring) method.
Up to now, examples of the fixing apparatus include a heating roller type and a film heating type.
The heating roller type employs a heating apparatus that heat-fixes an unfixed toner image to a surface of a recording material by including: a fixing roller (heating roller) and a pressure roller, which constitute a rotating roller pair; a heat source such as a halogen lamp built within the fixing roller so as to heat a portion to or control its temperature at a predetermined fixing temperature; and a press-contact nip portion (fixing nip portion) between the rotating roller pair through which the recording material as a heating material (a material to be heated), on which the unfixed toner image is formed and carried, is introduced, nipped, and conveyed.
The film heating type employs a heating apparatus that includes: a film having a small heat capacity instead of the heating roller so as to nip and convey a recording material between the film and the pressure roller; and a heater disposed to an inner surface of the film so as to heat the recording material. The film heating type increases the temperature for a short period of time, and is thus utilized as an on-demand fixing apparatus that can increase the temperature to a temperature for fixing, upon demand for heating to fix an image, immediately after starting the heating.
On the other hand, if the fixing roller or the film is coated with a resilient layer made of rubber or the like, the resilient layer functions as a heat insulating layer against the heating from the inside. Therefore, it takes longer even for the film heating type to increase the temperature, making it difficult to secure on-demand fixing.
In the case of coating a surface of a rotary member for heating a recording material as a heating material with a resilient layer, a releasing layer, or the like having heat insulating property, there is proposed an apparatus of a surface-heating type in which heat is supplied from a surface side of the rotary member (refer to, for example, Japanese Patent Application Laid-Open No. H10-133505). According to the apparatus of the surface-heating type, heat can be supplied from the surface of the rotary member for heating, improving responsiveness to power supply to a heater as a heating source. Even in the case of using the rotary member coated with the resilient layer, it is possible to reduce rising time.
However, in the apparatus of the surface-heating type, the rotary member has a temperature gradient that decreases from its surface to its deep part. Thus, even if the surface of the rotary member is deprived of heat due to insertion of the recording material as a heating material, there is little heat to be supplied from the inside of the rotary member. As a result, the amount of heat to be supplied to the recording material from the rotary member having a large amount of stored heat in the first revolution is different from that in the second revolution or later, causing a problem in that an uneven gloss (gloss non-uniformity) occurs due to uneven heating.
The above-mentioned circumstances have created a demand for a fixing apparatus with reduced rising time and without an image defect such as an uneven gloss occurring due to uneven heating.